Process and apparatus for collecting waste plastics as separated

Abstract: A process for collecting waste plastics as separated comprises the steps of presorting (1) out waste plastics from separated municipal refuse, crushing (2) the waste plastics, reducing the volume of expanded polystyrene included in the waste plastics by heating (3) the waste plastics at a predetermined temperature, finely dividing the waste plastics (4), washing the waste plastics to remove salt (5), etc. therefrom, separating the finely divided waste plastics in a liquid utilizing the difference between the plastics in specific gravity (6) and collecting the separated fractions, and drying (7) the useful one of the separated fractions.

Description: The present invention relates to a process for collecting waste plastics as separated, comprising the steps of: crushing waste plastics; reducing the volume of expanded polystyrene included in the waste plastics in a volume reducing device by heating the waste plastics in a predetermined manner, finely dividing the waste plastics; and separating the finely divided waste plastics in a liquid by utilising the difference between the waste plastics in specific gravity and collecting the separated fractions of waste plastics. Moreover, the invention relations to an apparatus for performing this process. Generally, waste plastics included in sorted or separated municipal refuse are discarded along with chinaware and other garbage for reclaiming land or are incinerated, whereass land reclamation involves the problem of securing the site of reclamation, and incineration involves the problem as to improvements of the incinerator. Accordingly, it is thought useful to exclude polyvinyl chlorides, which are not suited to conversion into oil, from waste plastics which are in the form of a mixture of various plastics and to convert the remaining plastics of higher grade (such as polyethylene, polypropylene and polystyrene) into oil for reuse. In converting waste plastics including polyvinyl chlorides, there arises the problem that the conversion of the waste into oil produces hydrogen chloride, deteriorating the conversion apparatus or impairing the safety of operation, or the problem of difficulty in handling bulky plastics such as expanded polystyrene. WO-A 93 14915 discloses a process for collecting waste plastics which comprises the steps of crushing waste plastics, reducing the volume of expounded polystyrene included in the waste plastics in a volume reducing device by heating the waste plastics in a predetermined manner, finely dividing the waste plastics and separating the finely divided plastics in a liquid by utilizing the difference between the waste plastics in specific gravity and collecting the separated fractions of waste plastics. In this process the crushing step, the volume reducing step and the dividing step reduce the volume of waste plastics to ensure ease of handling, and the separating-collecting step utilizes the specific gravity difference between the waste plastics to separate the waste plastics into fractions, i.e., a fraction of waste plastics, such as polyethylene and polypropylene, which are small in specific gravity and suited to conversion into oil, and a fraction of polyvinyl chlorides which are great in specific gravity and unsuited to conversion into oil, and to collect the separated fractions individually, whereby plastics of high grade are obtained which are suited to conversion into oil. From JP-A-1146708 it is moreover known to transport waste plastics in a volume reducing device by means of a screw-feeder while heating. However, the use of such a screw-feeder may lead to problems when the waste plastics to be treated are in the form of a sheet or a string, which may wind itself around the shaft of the screw and accordingly may not be treated with this device. Accordingly it is the object of the invention to provide a method and an apparatus of the above mentioned kind which are well suited for treating waste plastics in the form of sheets or strings. According to the invention this object is solved in that the process for collecting waste plastics in the volume reducing step the waste plastics are passed inside a trough by the rotation of a shaftless helical ribbon or shaftless spiral disposed inside the trough. Accordingly the apparatus of the invention is characterized in that the volume reducing device comprises a trough having an inlet at a starting end portion and an outlet at a terminal portion, a shaftless helcial ribbon or shaftless spiral disposed inside the trough, a drive unit for rotating the shaftless helical ribbon or shaftless spiral and a hot air generator for supplying hot air into the trough. Preferably the shaftless helical ribbon or shaftless spiral is made of a spring material. The hot air produced by the hot air generator is supplied to the entire interior region of the trough, for example, via a plurality of branch pipes arranged on the bottom of the trough at a specified spacing longitudinally thereof. The generator continuously supplies hot air into the trough so that the trough has an internal temperature preferably of 150 to 180° C, whereby the waste plastics inside the trough are heated while staying therein for 3 to 5 minutes. Expanded polystyrene softens and shrinks in this condition but will not melt together with other plastics, thus assuming a state favorable to the subsequent treatment. When to be treated by the volume reducing device, waste plastics are placed into the trough through the inlet and transported inside the trough from the starting end portion toward the terminal portion by the rotation of the shaftless helical ribbon or shaftless spiral while hot air is being supplied from the hot air generator to the trough. Of the waste plastics, expanded polystyrene and like plastics which are bulky, susceptible to the influence of static electricity and difficult to treat consequently have their volume reduced. Thus, the volume of bulky waste plastics can be continuously reduced by thermal shrinkage free of trouble. In addition, soiled plastics can be sterilized by heating to give a sanitary waste of reduced volume. Even when the waste plastics to be treated include those in the form of a sheet or string other than expanded polystyrene, the shaftless helical ribbon or shaftless spiral provided inside the trough makes it possible to continuously reduce the volume of the waste plastics without permitting the sheet or string to twine about the ribbon or spiral. Preferably, the separating-collecting device comprises a separating tank for separating a mixture of waste plastics into a fraction of low specific gravity and a fraction of high specific gravity, an inlet channel communicating with a bottom portion of the separating tank, a slurry tank for making the finely divided waste plastics into a slurry, and a pump for sending the slurry of finely divided waste plastics from the slurry tank to the inlet channel. With the separating-collecting device, the finely divided waste plastics in the form of a slurry are sent into the inlet channel by the pump. Even waste plastics of small specific gravity, such as expanded polystyrene, included in the charge are forcibly moved to the bottom of the separating tank by the pump pressure without being allowed to floatingly remain in the inlet channel, so that such light waste plastics will not interfere with the subsequent charge of waste plastics. Further since there is no need to produce a circulating flow by an agitator for feeding waste plastics to the separating tank, it is unlikely that bubbles formed by agitation will adhere to the surface of plastics. This eliminates the likelihood that polyvinyl chlorides of high specific gravity which are to settle in the tank will partly float inside the separating tank owing to bubbles adhering thereto. Consequently, the fraction of low specific gravity suited to conversion into oil can be collected as very accurately separated from polyvinyl chlorides which are unsuited to conversion into oil. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart showing a process embodying the invention for collecting waste plastics as separated; FIG. 2 is a diagram schematically showing the process and an apparatus embodying the invention for practicing the process; FIG. 3 is a perspective view partly broken away and showing an embodiment of device for reducing the volume of waste plastics; and FIG. 4 is a front view schematically showing an embodiment of device for separating and collecting finely divided waste plastics. Embodiments of the invention will be described below with reference to the drawings. FIGS. 1 to 4 show a process and apparatus embodying the present invention for collecting waste plastics as separated. The process for collecting waste plastics as separated comprises a presorting step 1 of separating waste plastics from separated municipal refuse, a crushing step 2 of crushing the waste plastics, a volume reducing step 3 of heating the crushed plastics at a predetermined temperature to reduce the volume of expanded polystyrene included in the waste plastics, a finely dividing step 4 of finely dividing the waste plastics, a washing step 5 of removing salt, etc. from the waste plastics, a separating-collecting step 6 of separating the finely divided waste plastics in a liquid utilizing the difference between the plastics in specific gravity and collecting the separated fractions of waste plastics, and a drying step 7 of drying the useful one of the separated fractions. In the presorting step 1, refuse 11 which has been collected as separated is separated into a plastics waste 12 and the other refuse (such as ashes, chinaware, glasses and rubbers) 13 manually. The refuse 13 other than the waste plastics is discarded. The waste plastics 12 sorted out are crushed in the crushing step 2 by a crusher 14 which is, for example, a two-shaft crusher. In the volume reducing step 3, the crushed waste plastics are wholly placed into a volume reducing device 16 from a hopper 15 and are heated, for example, at about 140° C for about 5 minutes. Expanded polystyrene which has a small bulk density and is difficult to handle has its volume reduced by this treatment and is made easy to handle. The heating is also effective for solidifying soy sauce, mayonnaise, jam, etc., rendering the waste easy to handle in the subsequent steps. If the heating temperature is higher than 140° C, expanded polystyrene will melt along with other waste plastics to become inseparable, so that the heating temperature is preferably about 140° C. FIG. 3 shows in detail a first embodiment of device for reducing the volume of waste plastics. The illustrated device comprises a trough 103 having an inlet 101 at its starting end portion and an outlet 102 at the terminal portion thereof, a shaftless spiral 104 disposed inside the trough 103, a drive unit 105 for rotating the spiral 104, and a hot air generator 106 for supplying hot air to the inside of the trough 103. The spiral 104 is made of a spring material and extends over the entire length of the trough 103 inside thereof. The hot air generator 106 mainly comprises an electric heater 109 having a control box 112 connected thereto, a fan 110 communicating with the inlet end of the heater via a flowmeter 113 and valve 114, and a hot air duct 111 communicating with the outlet end of the heater 109. The electric heater may be replaced by a heater of the burner type for use with kerosene. The duct 111 is provided with a plurality of branch pipes 108 arranged on the bottom of the trough 103 at a predetermined spacing longitudinally thereof. Each of the branch pipes 118 has upper and lower portions which are interconnected by a connector tube 115 of heat-resistant rubber for accommodating thermal expansion. The pipe has a valve 116 for controlling the rate of flow. The trough 103 and the hot air duct 111 are provided with a plurality of temperature sensors arranged at a specified spacing longitudinally thereof. These sensors feed to the control box 112 a signal indicating the detected temperature. The control box 112 controls the electric heater 109 in accordance with the signal to send hot air of required temperature into the trough 103. With the device described above, the hot air generator 106 produces hot air, which is continuously sent into the trough 103 over the entire length thereof so as to give an internal temperature of 150 to 180° C to the trough. Collected waste plastics, which are not limited to plastics of high grade and which are crushed, are placed into the trough 103 in this state through the inlet 101 and moved inside the trough from the starting end portion toward the terminal portion by the rotation of the shaftless spiral 104. Crushed plastics of high grade are prepared by removing polyvinyl chloride and polyethylene terephthalate from a mixture of plastics and crushing the remainder of high-grade plastics (such as polyethylene, polypropylene and polystyrene). The treatment in the trough reduces the volume of expanded polystyrene and like waste plastics included in the waste charge from about 0.03 to less than about 1.0 in terms of density, and sterilizes soiled plastics. The resulting waste is withdrawn from the outlet 102 and sent to the subsequent step. Even if the waste charge includes waste plastics in the form of a sheet or string other than expanded polystyrene, the spiral 104, which is shaftless, will not permit such plastics as softened to twine about the spiral 104. Although not shown, a second embodiment of volume reducing device has exactly the same contruction as the first embodiment except that the spiral 104 is replaced by a shaftless helical ribbon of spring material. The waste plastics given an increased density due to the volume reduction are finely divided in the finely dividing step 4 by a pulverizer 17 to sizes of not greater than 8 mm. The finely divided waste plastics are then sent through a cyclone 18 into a magnetic separator 19, in which metals are removed from the waste. In the washing step 5, salt, etc. (soy sauce, mayonnaise and the like) are removed from the waste plastics within a washing tank 20, and the waste is dewatered by a dehydrator 21. Washing water is supplied to the dehydrator 21 for rinsing, and the water discharged from the dehydrator is supplied to the washing tank 20. An excess of water overflowing from the tank 20 is sent to a waste water treating device (not shown). A separating-collecting device 8 is employed in the separating-collecting step 6 for separating the finely divided waste plastics utilizing the difference between the plastics in specific gravity and collecting the separated fractions. The device 8 comprises a slurry tank 22 for making the finely divided waste plastics into a slurry, a charge tank 24 having an agitator 29 and an opening for the slurry to be placed in therethrough, and a separating tank 23 for separating the mixture of finely divided plastics into a fraction of low specific gravity and a fraction of high specific gravity. The bottom of the separating tank 23 is in communication with the bottom of the charge tank 24. The separating tank 23 is filled with water. The water overflowing from the tank 23 is supplied to the washing tank 20 and the slurry tank 22. A circulating flow is produced by the agitator 29 within the charge tank 24 of the separating-collecting device 8, and the finely divided plastics waste is moved as entrained in the circulating flow into the separating tank 23 wherein substantially no flow is present. Within the separating tank 23, polyethylene, polypropylene and like waste plastics which are lighter than water and suited to conversion into oil float, while polyvinyl chlorides which are unsuited to conversion into oil are heavier than water and therefore settle. Accordingly, by collecting the floating fraction only, a finely divided plastics waste is obtained which is suited to conversion into oil and reduced in polyvinyl chloride content. The drying step 7 uses a dryer 25 for drying only the waste plastics suited to conversion into oil. These waste plastics 26 are sent to a conversion plant (not shown) for the subsequent process. The polyvinyl chlorides 27 settling in the separating tank 23 and not suited to conversion to oil are collected by a collector 28, solidified and discarded. Given below are examples of specific numerical values involved in the foregoing series of steps. The collected separated refuse 11 was 2.5 m 3 in volume and included about 90%, i.e., 2 m 3 , of waste plastics 12 weighing 47 kg. The waste plastics 12 were reduced in volume to 1.2 m 3 by the crushing step 2, to 0.5 m 3 by heating in the volume reducing step 3 and further to 0.12 m 3 by the finely dividing step 4. The hydrogen chloride concentration of the waste resulting from the finely dividing step 4 was about 6%, and about 10% when calculated as vinyl chloride. The plastics waste 26 eventually obtained and suited to conversion into oil gas 0.08 m 3 in volume, 30 kg in weight, 0.3% in hydrogen chloride concentration and 0.5% in water content. FIG. 4 shows an improved separating-collecting device for finely divided plastics. The illustrated device 200 comprises a separating tank 201 for separating the plastics in the form of a mixture into a fraction of low specific gravity and a fraction of high specific gravity, an inlet channel 202 communicating with a bottom portion of the separating tank 201, a slurry tank 204 for making the finely divided plastics into a slurry, and a pump 206 for sending the slurry of finely divided plastics from the slurry tank 204 to the inlet channel 202. In reusing waste plastics included, for example, in separated municipal refuse, the device separates polyvinyl chlorides which are unsuited to conversion into oil from the waste plastics to collect the plastics suited to conversion into oil. The separating tank 201 contains water such that polyvinyl chlorides greater than 1 in specific gravity settle therein while plastics, such as polyethylene arid polypropylene, which are smaller than 1 in specific gravity and suited to conversion to oil float. The slurry tank 204 is adapted to mix finely divided plastics with water to prepare a slurry of plastics and has an agitator 207 inside thereof. The inlet channel 202 has an inlet at the same level as the opening of the separating tank 201. The slurry of finely divided plastics is sent forward through a duct 205 and injected into the channel 202 through the inlet. The inlet need not always be positioned at the same level as the tank 201. The slurry injected into the channel 202 is forcibly moved to the bottom of the separating tank 201 by the pressure of the pump 206. At this time, expanded polystyrene and like plastics which are small in specific gravity are also moved with the other plastics and are unlikely to floatingly remain in the channel 202. Water flows at a low rate inside the separating tank 201, permitting each kind of plastics to exhibit characteristics in accordance with its specific gravity. Thus, a fraction 208 of low specific gravity such as polyethylene floats, while polyvinyl chlorides 209 of high specific gravity settle, whereby the plastic charge is separated. By collecting the floating fraction 208 of low specific gravity, plastics are obtained which are highly suited to conversion to oil and free from polyvinyl chlorides which are unsuited to conversion into oil. The settling fraction 209 of high specific gravity is separately collected as entrained in a circulating flow 203 produced by air lift, pump, screw or the like. With the improved separating-collecting device, expanded polystyrene and the like are unlikely to floatingly remain in the inlet channel 202 and will not interfere with the subsequent charge of plastics, hence a shortened treating time. Since there is no need to provide an agitator inside the inlet channel, hydrophobic polyvinyl chlorides are less likely to float owing to bubbles adhering to the surface thereof, and the device has another advantage of being compact. Table 1 shows the results of a separating and collecting experiment conducted using the improved separating-collecting device (device A) and the separating-collecting device shown in FIG. 2 (device B) for treating waste plastics including about 10% of expanded polystyrene. Plastics floating in inlet channel HCl content of floating fraction as collected Collection efficiency Device B 3.5% 2.0% 95.3% Device A 0.2% 1.5% 98.2% The table reveals that the device A is superior to the device B in any of the quantity of waste plastics floatingly remaining in the inlet channel 202, the amount of hydrogen chloride (amount of vinyl chloride) in the flating fraction as collected and the collection efficiency.

Claims: Process for collecting waste plastics as separated, comprising the steps of: crushing waste plastics; reducing the volume of expanded polystyrene included in the waste plastics in a volume reducing device (16) by heating the waste plastics in a predetermined manner, finely dividing the waste plastics; and separating the finely divided waste plastics in a liquid by utilising the difference between the waste plastics in specific gravity and collecting the separated fractions of waste plastics, characterized in that in the volume reducing step said waste plastics are passed inside a trough (103) by the rotation of a shaftless helical ribbon or shaftless spiral (104) disposed inside the trough. A process according to claim 1, characterized in that the waste plastics reduced in volume are finely divided into a slurry. Apparatus for collecting waste plastics (12) as separated comprising a device (14) for crushing waste plastics (12); a device (16) for reducing the volume of expanded polystyrene included in the waste plastics by heating the waste plastics (12) at a predetermined temperature; a device (17) for finely dividing the waste plastics (12); and a separating-collecting device (8, 200) for separating the finely divided waste plastics (12) in a liquid by utilizing the difference between the plastics in specific gravity and collecting the separated fractions of waste plastics (12), characterized in that the volume reducing device (16) comprises a trough (103) having an inlet (101) at a starting end portion and an outlet (102) at a terminal portion, a shaftless helical ribbon or shaftless spiral (104) disposed inside the trough (103), a drive unit (105) for rotating the shaftless helical ribbon or shaftless spiral (104) and a hot air generator (106) for supplying hot air into the trough (103). Apparatus according to claim 3, characterized in that the separating-collecting device (200) comprises a separating tank (201) for separating a mixture of waste plastics into a fraction of low specific gravity and a fraction of high specific gravity, an inlet channel (202) communicating with a bottom portion of the separating tank (201), a slurry tank (204) for making the finely divided waste plastics (12) into a slurry, and a pump (206) for sending the slurry of finely divided waste plastics (12) the slurry tank (204) to the inlet channel (202).

Inventors: ITO MICHIO, MIURA YOSHIMASA, OGINO ETSUO, TAKAKURA CHITOSE, ITO, MICHIO, MIURA, YOSHIMASA, OGINO, ETSUO, TAKAKURA, CHITOSE
Applicants: HITACHI SHIPBUILDING ENG CO, HITACHI ZOSEN CORPORATION
Application year: 1994
Main classification: B29B17
IPC codes: B29B17, B03B5, B03B9, B29B13